Molecular arrangement in two-dimensional streptavidin crystals

We studied the molecular arrangement of two-dimensional streptavidin crystals at the air-water interface over a range of pH values. We quantified the varying amounts of coexisting P1, P2, and C222 crystals in the different morphologies observed at pH 4.5-6.5. Chiral, needlelike crystals at pH 4.5 consist of P1 crystals with frequent line defects. Larger chiral domains near pH 5 are essentially all P1 coexisting with a small amount of P2, whereas at slightly higher pH values (near pH 5.5), II-shaped domains contain 4 times as much P1 coexisting with a P2/C222 mixture. Morphologies intermediate to these shapes exhibit intermediate compositions. Between pH similar to 6-7, crystals all display a characteristic dendritic-X morphology, but arrangement at the molecular level is quite different compared with lower pH values. Crystals are mostly P2 in symmetry near pH 6, but at pH 7 and above, crystals have C222 symmetry. Coexistence of P2 and C222 crystals occurs at intermediate pH values. We determined the orientation and arrangement of streptavidin molecules in P1, P2, and C222 crystals relative to the directions exhibiting faster growth. The direction of faster growth in P1 crystals includes both interactions between biotin-free subunits and interactions between biotin-bound subunits. In the P2 arrangement, growth in the direction of intermolecular biotin-free subunits is preferred, whereas growth is faster along the biotin-bound direction of C222 crystals. We developed a model of the molecular arrangement for the observed solid-phase coexistence in these crystals.